Final answer:
Tetrahedral molecular geometry refers to the three-dimensional arrangement where a molecule's central atom is surrounded by four electron groups oriented towards the corners of a tetrahedron, such as in methane (CH4). The ideal bond angle in such a geometry is 109.5°.
Step-by-step explanation:
The tetrahedral molecular geometry is a type of three-dimensional arrangement in which a molecule's central atom is surrounded by four electron groups. These electron groups could consist of atoms or lone pairs of electrons. In the perfect tetrahedral geometry, these groups are oriented towards the corners of a tetrahedron.
This configuration allows the groups to be as far apart as possible, minimizing the repulsion between them. An example of such a molecular shape is methane (CH4), where four hydrogen atoms are symmetrically distributed around a central carbon atom.
In molecules with a tetrahedral electron-pair geometry, the ideal bond angle is 109.5°, which is the angle between any two bonds in a perfect tetrahedron.
However, if there are lone pairs present, as seen in molecules like ammonia (NH3), the molecular shape becomes trigonal pyramidal, which is essentially a tetrahedron missing a vertex. The presence of lone pairs can lead to bond angles that are slightly less than the ideal angle of 109.5°.
For instance, in water (H2O), the H-O-H bond angle is reduced to approximately 104.5° due to the repulsion between the bonded pairs of electrons (BP) and the lone pairs of electrons (LP).